Editing Zipper monad/TravelBTree (section)

=== Moving around ===
There are five main functions for stringing together <hask>TravelBTree</hask> computations:

<haskell>
-- moves down to the nth child (0-indexed)
down :: Int -> TravelBTree a
left,  -- moves left a sibling
right, -- moves right a sibling
up,    -- moves to the node's parent
top    -- moves to the top node
:: TravelTree a
</haskell>

All five return the subtree at the new location. Note that <hask>down</hask> uses 0-indexed children, i.e. <hask>down 0</hask> goes down to the first child. This is consistent with the list-access operator, <hask>(!!)</hask>.